Any kind of electric or electronic device or element, either a signal-generating source or a load, needs power supply to work normally. However, each device or element requires different voltage. Please refer to FIG. 1 that shows a prior art. When a pulse-width-modulated (PWM) signal is in a duty cycle, it charges a capacitor C via a resistance R; and when the PWM signal is in a non-duty cycle, the capacitor C discharges until the PWM signal is Hi (that is, in the duty cycle) again. The capacitor C is then charged again. Therefore, generally speaking, an output voltage in the prior art shown in FIG. 1 is substantially constant.
However, in practical applications, the above-described prior art often fails to provide different voltages according to actual need. Therefore, it is necessary to improve the disadvantage in the prior art.